Electroluminescence elements, which uses electroluminescence (electroluminescence being abbreviated to “EL” hereinafter), have a high visibility because of spontaneous emission and further have good features, such as excellent impact resistance, because they are completely solid elements. Therefore, attention has been paid to the use thereof as light emitting elements in various display devices.
The EL elements are classified into inorganic EL elements, wherein an inorganic compound is used for their luminescent material, and organic EL elements, wherein an organic compound is used therefor. In particular, the organic EL elements are being developed as next-generation light emitting elements since the elements enable to reduce the voltage to be applied largely, easily enable the formation of full-color devices, are small in power consumption and make panel-emission possible.
An organic EL element basically has a structure of an anode/an emitting layer/a cathode, as illustrated in FIG. 3.
This organic EL element 10 has an emitting layer 14 sandwiched between a pair of electrodes composed of an anode 12 and a cathode 13. The emitting layer 14 is usually composed of plural laminated layers. When an electric field is applied across the electrodes 12 and 13 in this element 10, electrons are injected from the cathode 13 and holes are injected from the anode 12. The electrons and the holes are recombined in the emitting layer 14 so as to cause an exciting state. When the exciting state returns to a ground state, energy is emitted as light.
FIG. 4 shows an energy diagram of the organic EL element in FIG. 3. In FIG. 4, a valence electron level EV0 (HOMO) and a conduction level EC0 (LUMO), which are energy levels of the emitting layer 14, are shown. Holes go in the layer 14 from the anode 12 and electrons go therein from the cathode 13. The holes and the electrons are combined with each other in the layer 14 so as to emit light.
There is known an element wherein the above-mentioned structure is used as a base and a hole injecting-transporting layer and/or an electron injecting layer are appropriately added, an example of the element being an element having the following structure: an anode/a hole injecting-transporting layer/an emitting layer/a cathode; or an anode/a hole injecting layer/a hole transporting layer/an emitting layer/an electron injecting layer/a cathode.
The emitting layer has the following functions:
(1) Injecting function: a function capable of injecting holes into the layer from an anode or a hole injecting layer, and injecting electrons into the layer from a cathode or an electron injecting layer when an electric field is applied.
(2) Transporting function: a function of transporting the injected charges (electrons and holes) by the electric field.
(3) Light emitting functions: a function of supplying a field where the electrons and the holes are recombined and inducing light emission from this recombination.
The hole injecting-transporting layer has a function of injecting holes into the layer from an anode and transporting the holes into an organic emitting layer. A hole injecting layer and a hole transporting layer may be separately formed. The electron injecting layer has a function of injecting electrons into the layer from a cathode, and transporting the electrons into the organic emitting layer.
In order to make light emission in the emitting layer more intense, a technique of adding a very small amount of a fluorescent molecule (dopant) thereto is known.
FIG. 5 illustrates an energy diagram of an organic EL element to which a dopant is added. In this figure, ECh represents the conduction level of the host thereof; EVh, the valence electron level of the host; ECd, the conduction level of the dopant; and EVd, the valence electron level of the dopant. Egh and Egd represent the energy gap (difference between the ECh and the EVh) of the host and the energy gap (difference between the ECd and the EVd) of the dopant, respectively.
The dopant receives the energy of the excited host effectively so as to enhance the light emitting efficiency.
The method for obtaining multicolor from the luminescence of an organic EL element is roughly classified into the following three methods:
(1) A method of taking out lights of three colors from white luminescence emitted from an organic EL element by using three colors, red, green and blue filters.
(2) A method of converting blue luminescence emitted from an blue EL element to light of other colors by a fluorescent layer formed on the light-taking-out side of its emitting layer.
(3) A method of arranging blue, red and green emitting layers side by side on the same substrate.
For these methods, organic EL elements giving a high luminance and having a long durability have been demanded. For example, for the methods (1) and (2), the following has been earnestly desired: organic EL elements which emit light having a bluish green or white broad band (wide spectrum) and which have a half life of several ten thousands of hours or more for an initial luminance of several hundreds nit.
Various element structures have been investigated for such a desire.
As one method out of them, a method for making an organic emitting layer into plural layers, thereby obtaining white or broad band luminescence is suggested as follows:
(1) A way of making an organic emitting layer into a bi-layered structure, the first emitting layer in the anode side thereof being a blue emitting layer made of an aluminum complex compound, and the second emitting layer in the cathode side being a red emitting layer made of an aluminum complex compound containing a red fluorescent material, thereby taking out white light (see, for example, EP0643549).
(2) A way of making an organic emitting layer into a bi-layered structure, the first emitting layer in the anode side thereof being a blue emitting layer made of a distyrylarylene-based compound, and the second emitting layer in the cathode side being an emitting layer wherein a red fluorescent material is added to an aluminum complex compound emitting green light, thereby taking out white light (see, for example, U.S. Pat. No. 5,503,910).
(3) A way of incorporating coumarin and rubrene as dopants into an emitting layer which is a mixture layer of an electron transporting compound made of an aluminum complex and a hole transporting compound made of a diamine compound, thereby emitting light having a green component and an orange component (see, for example, WO98/08360).
(4) A way of doping a host material made of a distyrylarylene-based compound with two or more fluorescent materials different in color, thereby forming two emitting layers (see, for example, JP-A-12-68057).
These conventional techniques are ways for causing plural kinds of dopants equally to emit light to obtain broad band or white luminescence. The light emitted from the organic EL element is light wherein various wavelengths are mixed.
However, there is not known any organic EL element which has a multi-layered, laminated structure, emits light with a high color purity and a narrow band (narrow spectrum), and has a long durability.
In light of the above-mentioned problems, an object of the present invention is to provide an organic EL element which emits light with a high color purity and a narrow band and has a long durability.